Add configuration to publish hex package

[magnetised]

Claiming the electric package name for future fun.

• Copied the repo LICENSE file to the root of the Elixir app so it's present when installing as a dependency
• Removed the installation section from the README.md to avoid problems syncing the version
• Added a workflow step to publish the hex package alongside the NPM ones
• Fixed minor type issue (there's no such type as Stream.t())
• Note I had to publish pg_query_ex to Hex as well, because you can't publish hex packages with Github-based dependencies https://github.com/electric-sql/pg_query_ex/pull/4